1. Several plasmids containing inserts encoding single chain Fv (sFv)-myc- peptide fusion proteins have been produced by genetic engineering techniques. Two of these, anti-DNP sFv-myc and anti human transferrin receptor-myc, have been produced in bacteria, and give active protein capable of binding to cell surfaces and to target lysis in the presence of an anti-myc x anti-CD3 bispecific antibody. Other sFv constructs have been produced in COS cells. These are secreted directly into the medium as active proteins capable of binding to cell surface antigens. Fusion proteins of anti-CD3 and other sFv's are being constructed to see if a single chain bispecific antibody can be produced. 2. An sFv of the 2B4 murine T cell receptor has been produced. The 2B4 sFv is monomeric in aqueous solution and binds three mAbs that bind to the parental TCR. It specifically blocks antigen presentation to 2B4 cells at micromole concentrations, and specifically blocks superantigen mediated activation of T cells.